1. Field of the Invention
The present invention relates generally to a method of treating an arsenic-containing solution. It particularly relates to the treatment of an arsenic-containing brine in an open cooling tower and the like, in which a condensate of steam, derived from an arsenic-containing geothermal brine, is cooled.
2. Background Discussion
There are many sources of an aqueous arsenic-containing stream containing arsenic in a +3 oxidation state and for which there is a need to convert the arsenic to a higher oxidation state; for example, to convert arsenious acid to arsenic acid. The reason for converting the arsenic from a lower to a higher oxidation state is that the latter is far more reactive and, thus, more readily precipitated from the stream by reaction with a metal such as calcium or ferric iron to form a substantially water-insoluble, stable, calcium arsenate or ferric arsenate, respectively. Arsenic-containing fluids of interest are aqueous geothermal fluids. Of particular interest are the geothermal brines used for the production of electric power.
General processes by which geothermal brines can be used to generate electric power are well known. In a typical process, a geothermal brine at an elevated temperature and pressure is introduced into a zone of reduced pressure (flashed) to convert some of the brine into steam. The steam so produced is then used to drive a conventional steam turbine-generator combination to produce electricity. Generally, the remaining brine from the first reduced pressure zone will be introduced into one or more additional zones of further reduced pressure for the production of additional steam. Ultimately, the steam is condensed, recombined with any remaining brine and reinjected into a deep well for disposal.
Reinjection of the geothermal brine into an adjacent well is important to avoid the problems associated with the transport and disposal of the large amounts of saline and usually highly-contaminated liquid involved. Frequently, the brine contains small amounts of arsenic in the +3 oxidation state, most often present as arsenious acid. Arsenic in its +3 oxidation state is both water-soluble and highly toxic. Accordingly, any waste containing it must be disposed of in a hazardous waste disposal site. Obviously, the cost associated with disposal in a hazardous waste site is substantially greater than the cost for normal waste disposal.
One source of waste is the cooling tower and associated catch basin which are used to cool the steam. More particularly, a sludge forms in the cooling tower and the catch basin. The sludge comprises a mixture of living organisms, dust particles, precipitated salt from the brine and some entrained steam condensate. Many types of airborne organisms are known to grow at a very rapid rate in the hot, wet environment of the cooling tower and condensate catch basin. Such organisms include bacteria, algae, fungi and the like. While it is possible to control the growth of such living organisms, a significant amount of sludge will still form over an extended period of time. The sludge so formed must be periodically removed for disposal to prevent its interfering with the operation of the brine handling system. Since the sludge contains arsenic-containing condensate some treatment is required to permit disposal in a non-hazardous waste site.
Another problem associated with the use of a geothermal brine for generating electric power is that it is not always possible to reinject all of the brine into an adjacent well. For example, in some locations reinjection of all or even a significant portion of the brine produces a reduction in the temperature of the source brine. Another situation which can arise is that the reinjection well will not penetrate a subterranean formation having sufficient permeability to accept the brine at the same flow rate at which it is being processed. When reinjection of the brine is not possible, disposal of all or any substantial portion of the brine in a hazardous waste site would be prohibited by the cost. As an illustration, the production of only about 10 megawatts of electric power requires a continuous flow of over a millions pounds per hour of geothermal brine. When such large quantities of brine cannot be reinjected into a well, treatment to permit their disposal in a non-hazardous waste site is essential.
It is known that arsenic in its higher oxidation state is less toxic, for example, arsenic acid is less toxic than arsenious acid. It also is known that the higher oxidation state is more reactive and will react with, for example, iron and calcium to form stable, water-insoluble iron arsenate and calcium arsenate, respectively. A problem arises in that the materials taught by the prior art for the oxidation of arsenic require that at least a stoichiometric amount be used, and generally a slight excess is required to ensure substantially complete oxidation of all of the arsenic. In the case of geothermal brines, in addition to the arsenic, there also are many other ions which accept the oxygen as readily as arsenic and, since the oxidizers are not selective, they also will be oxidized. The net result is that a substantial amount of oxidizer is required. Clearly there is a need for a method of treating an aqueous solution containing arsenic in a +3 oxidation state to oxidize it to a +5 oxidation state without the necessity of using stoichiometric quantities of an expensive oxidizer.